Internal combustion engines, e.g., diesel engines, in which an oxidation catalyst and an adsorption catalyst are present for purifying the produced exhaust gas are known. In the case of gasoline engines, in particular direct-injection internal combustion engines, it is known that an adsorption catalyst can be used for purifying the exhaust gas. Such adsorption catalysts are operated in two operating phases. During a loading phase, the adsorption catalyst is loaded with nitrogen oxides, which are temporarily stored in the adsorption catalyst. During a regeneration phase, the nitrogen oxides are removed again from the adsorption catalyst, and the adsorption catalyst is regenerated in this manner. To regenerate the adsorption catalyst, i.e., during the unloading phase, an “enriched” exhaust-gas stream, i.e., an exhaust-gas stream loaded with fuel, is supplied to the adsorption catalyst. During the loading phase, the internal combustion engine may be operated with a lean mixture, i.e., at a lambda greater than 1. As mentioned, the nitrogen oxides produced in this case are temporarily stored in the adsorption catalyst and consequently do not reach the environment as pollutants. During the regeneration of the adsorption catalyst, the temporarily stored nitrogen oxides are converted into harmless substances and released into the open air.
In particular, in the case of diesel engines, it is known that a bifurcation element can be installed in the exhaust pipe, in order to accommodate a valve at this position. The volume of the exhaust-gas stream to the adsorption catalyst may be changed with the aid of the valve. In particular, the exhaust-gas stream to the adsorption catalyst may be decreased during the unloading phase, in order to prevent, in this manner, the adsorption catalyst from being acted upon too intensely by exhaust gases, and to simplify the enrichment of the exhaust-gas stream.
In diesel engines, it is also particularly well-known that, during the unloading phase, a fuel/air mixture can be added to the exhaust-gas stream supplied to the adsorption catalyst, in order to attain, in this manner, the already mentioned, enriched, and therefore fuel-loaded (lambda less than 1) exhaust-gas stream.
It has been shown that the above-described operating method, in particular the switching-over between the loading phase and the unloading phase of the adsorption catalyst, is not sufficient for preventing a release of pollutants to the environment to as large an extent as possible.
The object of the present invention is to provide a method for operating an internal combustion engine, which method minimizes pollutants released into the environment.